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PRODUCTION OF EXOTIC NUCLEI WITH RADIOACTIVE BEAMS AT FRAGMENTATION ENERGIES Kerttuli Helariutta, GSI Darmstadt
Foil 1. Title.
Foil 2. Example of a beam-target combination 238U + 208Pb, which results in a several types of possible reactions with wide energy range:
In the figures, cross sections range from 100 mb to 10 m b. Upper figure – experimental cross sections, T. Enqvist et al., Nucl. Phys. A 658 (1999) 47. Lower figure – the cross sections for the same reaction, calculated with ABRABLA code. In general, the agreement between the experiment and the model is better than a factor of two. Therefore, one can think that the lower figure represents the whole actual reaction product distribution. Despite of the multitude of reactions and the varying energy range, no new neutron-rich areas are populated significantly.
Foil 3. The basic properties of the fragmentation reaction.
Foil 4. Cold fragmentation. It has been observed experimentally that there are fragmentation reactions where only protons are removed and the energy of the formed fragment is so low that no neutron evaporation follows the fragmentation. (J. Benlliure et al. Nucl. Phys A 660 (1999) 87)
Foil 5. Two-step reaction scheme. It has been suggested that the cold fragmentation could be combined with fission in order to reach the very neutron rich areas. The idea is to first have a fission reaction (e.g. p + 238U), choose an abundant fission product (e.g. 132Sn) and then re-accelerate and fragment it. We have done some model calculations on the feasibility of this scheme (J. Benlliure et al. GSI-Preprint-41, Nov. 2000). The first step (the fission) is well known experimentally, thus we focussed our calculations to the second step (the fragmentation of a very neutron-rich projectile).
Foil 6. The computer codes that were used in our calculations:
Foil 7. An example of our calculations. The cross sections for producing 124Pd via fragmentation of different projectiles were calculated. As a reference point, an experimental value of the cross section of 124Pd from the fragmentation/fission of 238U was used (M. Bernas et al., Phys. Lett. B 415 (1997) 111). In calculations, two projectiles close to the wanted fragment were chosen: 136Xe (stable) and 132Sn (radioactive). Results:
Foil 8. Conclusion. According to our study, the two-step reaction scheme looks promising, supposing that the secondary beam intensity is high enough. A care must be taken when using different models for predicting cross sections for the fragments from the very neutron-rich projectiles: semiempirical systematics like EPAX may not be valid any more.
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